1. Field of the Invention
The present invention relates to an improvement of an image heating apparatus for an image furnace which is employed for crystal growth of, for example, a semiconductor material.
2. Description of the Prior Art
FIG. 1 is the sectional view illustrating an image heating apparatus of a prior art as disclosed, for example, in "Solid State Physics" vol. 14, No. 10, 1979, pp 633-640. In FIG. 1, numeral 1 designates a rotary elliptical mirror having a reflective surface of a rotary elliptical body at the inside, numeral 11 a light source comprising such lamps as halogen lamp, xenon lamp or the like disposed at the first focal point of the rotary elliptical mirror 1 and adapted to emit the light 9, numeral 12 a power source connected to the light source 11 via a wiring 18 for supplying the electric power, and numeral 8 a test sample positioned at the second focal point of the rotary elliptical mirror 1.
Since the image heating apparatus according to a prior art was constituted as described above, the light emitted from the light source was reflected by the rotary elliptical mirror 1 and condensed on the surface of the test sample 8. Accordingly, the temperature of the test sample 8 can be raised and the test sample may be melted to produce crystal.
However, since a halogen lamp or a xenon lamp is used for the light source, the emitting range of the light source is as small as 5 mm. As a consequence, a small image is focused at the surface of the test sample 8. This causes an acute temperature gradient, such that cracks might be developed in the test sample 8. It is therefore necessary, in the case of the image heating apparatus of a prior art, to adjust the heating area of a test sample depending on the requirement. This adjustment is made in such a way as to displace a part of the rotary elliptical mirror so as to blur the focus or prepare an after heater to complement the blur the focus or prepare an after heater to complement the heating. Provision of an additional mechanism as above will considerably degrade the operational efficiency of a furnace and make the constitution of the apparatus more complicated, impairing the function of the furnace. Thus, a serious problem ensued.
Furthermore, when such lamps as halogen lamp, xenon lamp or the like having electrodes are employed as the light source, their relatively short life [for example, in the order of about 40 hours (in the case of application for space technology)] inevitably required a large number of spare halogen lamps to be stocked. In case of halogen lamps or xenon lamps, since it was not possible to directly melt the glass by its light, and the glass was placed in a mesh of platinum material and subjected to image heating, the platinum material intruded into the glass when it was melted, thus degrading the quality. This caused a big disturbance, particularly in the course of processing such materials as infrared fiber or the like.
Furthermore, since only a narrow range is heated by the light source of a prior art, uniform heating in the circumferential direction of a test sample is not possible. For this reason, the test sample has to be rotated at a speed as high as 100 r.p.m. cause uniform distribution of temperature.
However, if the test sample is rotated at such a high speed, uniform crystal growth may be disturbed. Particularly in the case of a microgravity experiment, since artificial gravity is created and air bubbles are enclosed in the crystal by the centrifugal force and can never come out, which is a serious problem. This problem has not yet been solved worldwide. Therefore, for the image furnace to be used in a space shuttle experiment, the test sample is rotated at a high speed so as to attain a uniform heating. In this respect, it is apprehended that the microgravity effect may not be adequately performed.